This invention relates to articles having wear resistant surfaces and more particularly to tools having diamond coated wear resistant surfaces. In one form of the invention it relates to cutting tools and more particularly to diamond inset cutting tools.
In this specification, unless the context clearly indicates otherwise, the expression "articles having wear resistant surfaces" include bearings, sliding parts, articles subjected to wear as a result of the slipping of other elements thereover such as textile bobbins and cutting tools. The expression "cutting tool" is used in a general sense to denote any kind of tool which, in use, performs essentially a cutting action, at least when viewed at the microscopic level, by slicing or otherwise removing a piece or successive layers of material from a workpiece. Accordingly, the expression is intended to include, but not to be limited to, such equipment as saws, knives, drills, reamers, files, burrs, bits and surfaces coated for grinding application e.g. grinding wheels. The expression "diamond coated surface" as used in this specification is intended to cover both "diamond inset" surfaces as defined below and is specifically intended also to embrace surfaces onto which a coating of artificial diamond particles had been formed by the industrial process known in the trade as chemical vapor deposition [CVD] with or without plasma and/or microwave assistance. Such polycrystalline diamond nuclei formed by this deposition process may be formed on any suitable metallic or ceramic substrate including silicon carbide, tungsten carbide or natural diamond. The diamond coating may be of any required thickness but is typically a few microns thick. No bonding material is used for such CVD produced diamond coatings. Typically, we refer to the application of the 2,45 GHz microwave plasma CVD process in which a mixture of hydrogen and methane [1%] is used at a total gas flow rate of 100 cc/min, total pressure of 40 Torr, microwave power of 200 to 300 Watt and the substrate temperature between 880.degree. C. and 980.degree. C. A non-continuous layer of polycrystalline diamond nuclei, each nucleus about 10 microns in diameter, is thereby deposited on a WC substrate 1 mm diameter drill.
The expression "diamond inset" is herein intended to refer to any form of mounting diamonds or diamond dust particles on the working surface of any wear resistant article including any cutting tool and accordingly includes randomly scattered diamond dust particles held on the tool surface by being partially embedded in a matrix material. "Matrix material" is defined to be the bonding material such as nickel used when natural and artificial diamonds require bonding but it must also be read to refer to the substrate when CVD produced diamond coatings are referred to.
Diamond inset cutting tools are known to be used in various trades ranging from metal working equipment to surgical and dental drills and burrs. Such equipment generally comprises a working surface constituting a surface matrix into which the diamonds or diamond dust particles are set and well bonded. The matrix composition depends on the application of the tool in question but typically comprises a nickel-based alloy containing such elements as phosphorous, cobalt, wolfram and/or silicon. It is also known to use both natural and synthetic diamonds or a mixture thereof in producing diamond set cutting tools of the type in question.
The recently developed CVD produced polycrystalline diamond coated surfaces without any bonding has not yet found application as cutting tools due to the generally experienced and reported lack of adhesion of such produced coatings.
Although diamonds are very hard, diamond coated cutting tools wear relatively rapidly. It is, for example, the experience of dentists that a diamond coated burr may have a useful life allowing it to be used for tooth preparation of approximately 6 to 8 teeth. The wearing of such burrs and of other diamond set cutting tools is associated with two distinct processes. Firstly, the diamond dust particles may become dislodged from its mounting in the metallic matrix and secondly, the dust particles may be fractured along cleavage planes or fractures when impacted by the workpiece, such as a tooth, or by debris created during the working process. Such deterioration by particle loss and fractures in diamond burrs used in dental practice has been well demonstrated by Ir A. N. Westland in "The Energy Requirement of Dental Cutting Process" Journal of Oral Rehabilitation, 1980, Vol. 7, pp. 51-63.
Electronmicroscopic studies by the present applicants of the particles of diamond dust used on dental burrs have revealed that the dust particles often exhibit incomplete microfractures. It is believed that these incomplete microfractures constitute weak zones in the diamond dust particles and it is accordingly to be expected that the particles are in use prone to be fractured along such zones. Such fractured particles, along with dislodged particles, give rise to the presence of relatively large, very hard, loose bodies forming an abrasive grit in the working area which not only leads to the further fracturing or dislodgement of particles, but also leads to the erosion of the matrix thereby releasing, inter alia, nickel to the surrounding area. Nickel is a highly toxic element and its release during dental or bone surgery is accordingly most undesirable.